1. Field of the Invention
The present invention relates to a thermoelectric semiconductor material and a method for manufacturing same.
2. Description of the Related Art
A thermoelectric cooling element which uses Peltier effect or Ettingshausen effect or a thermoelectric power generation element which uses Seebeck effect is attracting attention in the point that it can be used over the wide range because it has a simple structure, is handled easily and keeps stable characteristics. Particularly, as a thermoelectric cooling element can control a local cooling and a precise temperature in the vicinity of room temperature, its extensive research and development are being made for its application to optoelectronics, temperature control of semiconductor lasers, a small refrigerator and the like.
A material of the thermoelectric element used for the thermoelectric refrigeration and thermoelectric generation electricity has a figure of merit Z(=.alpha..sup.2 /.rho..kappa.) represented by Seebeck coefficient .alpha., electrical resistivity .rho. and thermal conductivity .kappa., which are peculiar constants of the material in a range of temperatures it is used.
Specifically, a crystal material generally used for thermoelectric cooling element and the like includes mixed crystal such as bismuth telluride (Bi.sub.2 Te.sub.3), antimony telluride (Sb.sub.2 Te.sub.3), and bismuth selenide (Bi.sub.2 Se.sub.3).
Such Bi.sub.2 Te.sub.3 based semiconductor materials, e.g., bismuth telluride (Bi.sub.2 Te.sub.3) and antimony telluride (Sb.sub.2 Te.sub.3), have a layer structure as shown in FIG. 10. Specifically, Te, Bi (or Sb), Te, Bi (or Sb) and Te are bonded sequentially from top to bottom into a five-layer form by covalent bonding. And, three of such five-layer forms are bonded into one 15-layer unit by Van der Waals bonding or ionic bonding so to form a stratified layer crystalline structure.
Meanwhile, a PbTe-based or PbSnTe-based semiconductor material is used as a thermoelectric conversion material used in a middle temperature range (from room temperature to 400.degree. C.).
Such PbTe-based and PbSnTe-based semiconductor materials have high mobility of a carrier in the crystal. This semiconductor material provides a remarkable thermoelectric property when it is in a monocrystalline state or formed into a polycrystalline ingot material by solidifying in one direction.
But, since its material strength is low for industrial use, it is necessary to improve the strength by forming into a sintered body.
However, the sintered body has an increased resistance and a lowered thermoelectric property because an influence of scattering in a grain field due to high carrier mobility.
As described above, the crystal material generally used for the thermoelectric cooling element or the like includes mixed crystals such as bismuth telluride (Bi.sub.2 Te.sub.3), antimony telluride (Sb.sub.2 Te.sub.3), and bismuth selenide (Bi.sub.2 Se.sub.3).
Conventionally, a method for synthesizing a compound consisting of a plurality of elements as described above was a "melting and solidifying method" which seals raw material elements into a glass tube, melts them to synthesize in a liquid phase, and cools to solidify.
The above melting and solidifying method may not provide a uniform phase because two phases are formed depending on the composition of the compound during solidifying by simply cooling a liquid phase.
For example, it is assumed that compound PbBi.sub.2 Te.sub.4 is synthesized.
The compound PbBi.sub.2 Te.sub.4 is a compound having PbTe and Bi.sub.2 Te.sub.3 mixed at a stoichiometric ratio of 1:1. The phase diagram of this PbTe-Bi.sub.2 Te.sub.3 alloy is shown in FIG. 12. It is apparent from the drawing that when PbBi.sub.2 Te.sub.4 is synthesized by the melting and solidifying method, a compound having a uniform structure cannot be obtained because it is separated into a compound which includes PbTe excessively than the target composition PbBi.sub.2 Te.sub.4 and a compound which includes Bi.sub.2 Te.sub.3 excessively than the target composition PbBi.sub.2 Te.sub.4.
Specifically, a composition of 50 molt % Bi.sub.2 Te.sub.3 -50 molt % PbTe is indicated by straight line 1 in FIG. 12. The solid PbBi.sub.2 Te.sub.4 compound is formed when a temperature is below horizontal line 5 on the straight line 1. Then, when a solute, which was prepared by weighing to have the aforesaid composition and dissolving, is cooled down to temperature T.sub.1 (melting point) at point A where the straight line 1 intersects with liquidus line 2, a solid of composition 4 corresponding to point B (temperature T.sub.1) on solidus line 3 is precipitated from the solute.
When the temperature is further lowered, the composition of the solute changes from point A to point C, and the composition of the precipitated solid changes from point B to point D. Upon reaching temperature T.sub.2 of the horizontal line 5, the solute of the composition at point C and the solid of the composition at point D are present at the same time. At temperature T.sub.2, the solute at point C reacts with the solid at point D. If temperature T.sub.2 can be maintained for a sufficient period of time, the solute at point C and the solid at point D are fully reacted, and the compound of composition PbBi.sub.2 Te.sub.4 at point C can be formed uniformly. But, it is practically difficult to keep a fixed temperature for a very long period, and a compound having a different composition is precipitated due to a little change in temperature. As a result, a solid obtained by cooling contains the compound having the composition in the vicinity of point C and the compound of the composition in the vicinity of point D in addition to the desired PbBi.sub.2 Te.sub.4 compound.
Accordingly, for such a compound which cannot be provided with a uniform phase by the melting and solidifying method, a zone melting method or a zone leveling method can be used.
But, an apparatus for conducting the zone melting method or the zone leveling method is complex and expensive. And, an obtained ingot often has a portion with a composition different from the target composition, resulting in a poor yield.
Meanwhile, a solid phase reaction which synthesizes at a temperature lower than the melting point may be applied in order to prevent the separation into two phases at the time of soldering.
But, a long period of tens of hours is necessary to fully complete the reaction by the solid phase reaction. Thus, a production efficiency becomes poor and a production cost is high.